McCusker studied acting at the Boston Conservatory of Music, graduated from the Neighborhood Playhouse, [1] and started her acting career in the early '70s when she had guest roles in the television series Play for Today (1971, with Maurice Roëves). In the following years she had supporting roles in the comedy Tunnel Vision (1976, with Gerrit Graham and William Schallert), the comedy Cracking Up (1977, with Michael McKean and Leslie Ackerman), the thriller The Border (1979), and the television drama The Ordeal of Patty Hearst (1979, with Jonathan Banks and Star Trek voice casting agent Barbara Harris).

In the '90s, McCusker focused on her acting coach career and appeared in a handfull of productions, including Joel Schumacher's thriller The Client (1994), the drama The Cure (1995), the Doctor Finlay episode The Life of Men (1996), and the comedy Trojan War (1997, with Allan Graf and Jake Sakson).

McCusker has worked as acting and dialogue coach for several teenage and child actors such as Seth Adkins in ...First Do No Harm (1997), Malcolm Stumpf in The Next Best Thing (2000), and for the Jonas Brothers on the television pilot J.O.N.A.S! (2009). Other acting coach credits include the thriller The Client (1994), the comedy Operation Dumbo Drop (1995), the drama Tom & Huck (1995), the thriller Sleepers (1996), the comedies Paulie (1998) and Jane Austen's Mafia! (1998), and the television series Malcolm in the Middle (for seven years), The Bernie Mac Show, and Nip/ Tuck.